The invention relates to improvements in liquefied gas pumps, and more particularly to improvements in apparatus for conveying liquefied gases wherein the conveying operation is carried out by one or more multi-stage centrifugal pumps. Still more particularly, the invention relates to improvements in apparatus wherein at least the first stage of the multi-stage centrifugal pump is immersed in a supply of liquefied gas which is confined in a vessel, such as a barrel or a can.
Apparatus of the above outlined character are normally equipped with mechanical seals which operate between the shaft and the housing of the multi-stage pump and are acted upon by a suitable sealing liquid to prevent escape of the conveyed medium along the shaft at that end of the pump housing which is not immersed into the supply of liquefied gas in the vessel. It is also known to provide the housing of a multi-stage pump in an apparatus for conveying liquefied gases with various means, including flow restrictors and chambers, for preventing the penetration of liquefied gas all the way to the shaft seal.
An apparatus which is to convey a liquefied gas must be designed in such a way that it will operate properly even if the pressure of liquefied gas at the inlet of the centrifugal pump merely matches the saturation pressure. However, and since a centrifugal pump will operate properly only if the pressure upstream of the impeller in its first stage exceeds the saturation pressure of the conveyed medium, it is necessary to rely on special undertakings in order to ensure that the pressure of the conveyed medium ahead of the impeller in the first stage of the multi-stage pump will invariably exceed the saturation pressure. This is the reason for placing at least the first stage of the pump into a vessel which contains a supply of liquefied gas and wherein the upper level of such supply extends above the first stage. The pressure of the column of liquefied gas above the first stage ensures that no cavitation will develop in the region of the impeller of the first pump stage.
In many instances, the centrifugal pump of an apparatus for conveying a liquefied gas is an upright pump wherein the first stage is located at the lower end of the pump housing. As a rule, the apparatus consititutes a so-called barrel pump or can pump wherein the barrel or can is a vessel which receives the lower portion of the pump housing. The utilization of a barrel pump or can pump for conveying a liquefied gas brings about certain other problems which still await a satisfactory solution. Thus, the pump shaft is located in the interior of the standpipe which, in turn, renders it necessary to design the shaft seal at the upper end of the pump housing in such a way that the seal will operate properly in spite of the fact that the pressure at the outer side of the seal equals atmospheric pressure but the pressure at the inner side of the seal can reach the maximum pressure of the pumped fluid medium. This is the reason for the utilization of mechanical shaft seals. If the maximum pressure of the conveyed medium is very high and/or if the mechanical seal is operated with a sealing liquid which should be prevented from contaminating the conveyed medium, it is necessary to ensure that the mechanical seal will be contacted only by the gaseous phase but is invariably kept out of contact with the liquefied gas. Attempts to accomplish this include the provision of complex flow restrictors and large expansion chambers for liquefied gas downstream of the flow restrictors. Expansion of the liquid phase in a large chamber entails its conversion into the gaseous phase. Each expansion chamber must be connected with a space wherein the pressure is relatively low so that it can receive the large quantity of gaseous phase which develops in the expansion chamber. The need for large expansion chambers has prevented this proposal from gaining acceptance in the relevant industries, such as refineries, natural gas plants and certain others. Attempts to admit the gaseous phase into the inlet of the centrifugal pump have failed in view of the large quantities of gases. The proposal to admit the gaseous phase into the vessel for the pump housing is not acceptable because it renders it necessary to provide a large number of pipes which contribute significantly to the initial and maintenance cost.
In accordance with another prior proposal, the direction of flow of conveyed medium through the barrel pump is reversed. The pump is installed in the lower part of the vessel and receives liquefied gas through a downwardly extending conduit. The liquefied gas is boiling at the locus of entry into the centrifugal pump and, therefore, it presents no problems to ensure that the shaft seal is contacted only by a gaseous phase. However, the pressurized liquefied gas which issues from the centrifugal pump must enter the vessel which, therefore, must be designed to stand the elevated pressure of the pumped liquefied gas. This renders it necessary to employ high-quality materials and to increase the bulk and weight of the vessel beyond acceptable levels. An additional problem in connection with the utilization of such apparatus is that the bearings which are immersed in liquefied gas are continuously contacted by a boiling fluid which prevents adequate lubrication. Improper maintenance of bearings entails vibrations of the rotor and of the conduit which admits a descending column of liquefied gas into the first stage of the centrifugal pump. Such vibrations lead to cracking of the material and to outright breakage of the respective parts which, in turn, entails a slowdown or complete stoppage of the plant.